None of the photographs on this post are the product of PhotoShop manipulation. So what’s the blue luminescence you see around these swimmers and kayakers? That’s the result of the natural properties of Puerto Rico’s Laguna Grande.

The Laguna Grande—colloquially referred to as the “Bioluminescent Bay” or simply the “Bio Bay”—is a relatively small and shallow body of water with a very narrow opening to the ocean and an extensive mangrove forest at its edge. All of those qualities combine to produce an ideal environment for dinoflagellates to thrive.

No, a dinoflagellate has nothing to do with a dinosaur fart. Instead, it is a one-celled organism (see photo, right). Each dinoflagellate has a whip-like tail that enables it to move through the water and a bulbous body that lights up when it is agitated. Scientists are not sure what evolutionary advantage is gained by this lighting up—maybe it is a method dinoflagellates use to warn each other of danger, maybe it is their way of amusing tourists. In any event, if you put your hand in the Laguna Grande and swirl it around you’ll cause the water to light up.

Planet Earth has a few other bays and lagoons where turning up the waters will consistently cause a glowing effect. None of them are as amazing as the Laguna Grande. The bay’s superlative bioluminescence comes thanks in part to the 17th-century Spanish explorers who placed rocks at the mouth of the bay in an attempt to choke it off from the ocean’s water. They did this in an effort to prevent the bioluminescence, which they somehow concluded was the work of the devil. Ironically, by reducing the flow of ocean water the Spanish increased the concentration of vitamin B12 that the mangrove tress put into the bay, which increased the dinoflagellate’s food source, which increased the number of dinoflagellates and the glow they produce. A typical gallon of Bioluminescent Bay water contains 750,000 dinoflagellates—the highest naturally occurring concentration of any body of water in the world.

The Bioluminescent Bay is off the coast of island of Vieques, about ten miles from the Puerto Rico mainland. Between 1941 and 2003 two-thirds of Vieques was used by the United States Navy for target practice. Tourists and real estate developers, both of whom generally dislike being bombed by battleships, stayed away. When the navy finally decided to abandon Vieques they converted their property there to a wildlife and nature sanctuary. So, in a wonderful example of unintended consequences, Vieques’s war history has today made it one of the most beautiful and untouched areas of the Caribbean.

Whether you plan on churning up the waters of the Bioluminescent Bay or not, Vieques is a stunning travel destination.

If you go: Swimming is no longer allowed in the Bioluminescent Bay. You can, however, go kayaking; several local providers run night time tours. To get the full experience of the bay definitely visit on a moonless night. A cloudy night within a few days of the new moon will work too, but you’ll be disappointed if you go during a full moon. Click here for a moon phase calendar.

If you can’t go: Consider creating a very much miniaturized version of the Bioluminescent Bay in your bathtub. Click here for instructions on how to grow your own dinoflagellates at home.